Environmental sampling and analysis is regularly performed to determine the amount of various target compounds are present an area such as a chemical facility, a laboratory, confined space, or other area which could potentially be contaminated by chemical compounds. Samples may be collected over a short sampling period of time indicating a potential “instantaneous” exposure of personnel to the compounds (“grab sample”) or the sample may be collected over an extended sampling period of time to determine a potential average exposure of the personnel to the compounds over the extended sampling period of time. Conventionally, samples are typically drawn into a sample bag by a sampling pump or collected on a cassette, sorbent tube, or impinger attached to the pump.
There are basically two conventional sampling systems, direct sampling systems and indirect sampling systems. Conventional direct sampling systems comprise a pump to move the gas to be sampled into a sample container, conversely, conventional indirect sampling systems use a vacuum to expand the sampling bag to draw a gas to be sampled into the sample container.
These sampling systems are designed to relatively easily obtain an accurate whole air sample and minimize contamination of the sample during the sampling process and during storage of the sample. Most sampling systems include an inflatable sample bag used to collect and/or store air, vapor, and/or gas samples or are used to collect liquid, air, vapor, and or gas samples on a sorbent tube, cassette, impinger, or other collection media by drawing the liquid, air, vapor, and or gas sample through a sample bag inlet, the sorbent tube, cassette, and/or other collection media when the sample bag is inflated or a pump is activated.
Sampling in confined spaces provides additional concerns and difficulties. The Occupational Safety and Health Association require that in some instances before an employee enters the confined space, the atmosphere within the confined space should be tested for oxygen content, for flammable gases and vapors, and for potential toxic air contaminants.
When monitoring for entries into a confined space involving a descent that comprise a stratified environment, the atmosphere should be tested a regular distances in the direction of travel and to each side. If a sampling probe is used, the entrant's rate of progress into the confined space may have to be slowed to correspond with the sampling speed and detector.
Typically to sample in a confined space, a remote sampling tube is attached to a pump in a sample analyzer. The open end of the tube is lowered into the confined space and the pump is activated to pull a sample. There are several disadvantages with this system. For example, in this system the tube is very long and it is difficult to determine if the tube is entirely purged so that the sample being taken is representative of the environment at the end or inlet of the tube. Further, certain chemical compounds may be absorbed on the walls of the tube during the sampling process thus changing the concentration of the sampled gas as it moves through the tube to the sample bag or analytical device. Further, the tubing may comprise adsorbed compounds from previous sampling processes that are desorbed into and contaminating the current sample. In addition, when a volumetric pump is used, the sampling tube and the associated resistance can result in less sample volume being drawn into the sensing device than anticipated during the specified sampling period.
There is a need for improved means of sampling in confined spaces.